In day, when you look in the room through the window out, you can clearly see what happens outside. At night when it's dark outside but there's light inside you can look in the window but it becomes a mirror.
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Any translucent surface both reflects and refracts light. By refraction, I mean that it bends the light a bit, but lets it through to the other side. Now, reflection for such surfaces is much less than refraction (unless there's total internal reflection, but thats irrelevant for glass+air). Edit: According to @JohnRennie (see comments), only 5% of the light is reflected
During the day, you have light from your room being largely refracted out, and reflected back inwards a tiny bit. The outside light does someing similar. It is largely refracted into your room, and reflected back outside a tiny bit. So, the majority of the light you see coming from the window is due to the outside light. You will see a reflection if you look carefully (exacly how carefully depends upon the lighting of your room)
Now, during the night, there is little or no light coming from the outside. So the majority/all of the light you see is due to reflection. So you see the reflected image.
Now an interesting question is, if the reflected image has the same intensity in both cases, why do you see it in one case and not see it in another? The answer lies in the working of the eye. The eye does not have a constant sensitivity to light. Whenever there is a lot of light, your irises contract, admitting less light into your eyes. This means that you can perceive bright light but dim light becomes invisible. When it is darker, they expand, and the reverse effect happens. That's why you feel blinded by bright light when you leave a dark room, and also why it takes time to adjust to a dark room. (You can actually see your irises contracting; go to a well lit room with a mirror, stare at your eyes, close them for a few seconds, then reopen.. Takes a few tries, but you can see them contracting). Edit: (Credit @BenjaminFranz for pointing this out) The regulatory mechanism does not consist of only the iris/pupil. The retina also does a lot of regulation, which is why it takes half a minute or more to get used to a dark room, whereas our irises can dilate within a few seconds.
So, during the day, the profusion of light refracted from the outside makes your irises contract, thus making the reflected light nearly invisible. During the night, your pupil is dilated, so you can clearly see a reflection.